I believe the result is that atheists have an above average knowledge of world religions, similar to Jews (and Mormons) but I don't know of results that show they have an above average knowledge of their previous religion. Assuming most of them were Christians then the answer is possibly.

In this particular case I happen to know precisely what is in all of the official church material; I will admit to having no idea where his teachers may have deviated from church publications, hence me wondering where he got those beliefs.

I suppose I can't comment on what the average believer of various other sects know of their sects beliefs, only on what I know of their sects beliefs. Which leaves the question of plausibility that I know more then the average believer of say Catholicism or Evangelical Christianity or other groups not my own.

[edit] Eliezer, I am not exactly new to this site and have previously responded in detail to what you have written here. Doing so again would get the same result as last time.